There is a free version with a limited feature set available for personal use. Unfortunately, some essential features are missing (like tags and thought/link types), but at least you are not limited in the number of thoughts (= nodes) you can create, so it may be good enough for some people.

I’ve been using TheBrain (previously called “PersonalBrain”) for more than a decade now and got a lot more value out of its graph-based navigation and spatial linking concept than I would have with any of the text/outliner-based tools that have been popping up recently (although I use some of them too, for different reasons).

It's annoying that one time pricing feels worse than recurring. Roam is $15/month, so much cheaper!